In pulp industry, recovery furnaces are used as a chemical reactor and for the production of steam for internal use, for generation of electricity, and for sale. As the recovery furnace operates as a chemical reactor, the combustion conditions differ from those of an ordinary boiler, in that the heating surfaces of the furnace get covered extremely rapidly with combustion deposits, i.e. slag, ash and/or soot, which decrease the efficiency of the recovery furnace, particularly by reducing heat transfer in the furnace. In addition to soot, the flue gases contain inorganic chemicals, which condense on the heating surfaces of the recovery furnace.
Recovery furnaces require continual cleaning of the heating surfaces by means of special cleaning apparatus, called sootblowers. The sootblowers clean the heating surfaces with high pressure steam, and generally about 2-10% of the steam production of the furnace is used for cleaning the recovery furnace. If the time between successive cleanings is too long, the dust-like particles get harder and/or sinter, and the deposits will be harder to remove.
Generally, the sootblowing system comprises about 40-80 sootblowers and is very expensive subsystem of a recovery furnace. As a rule, each individual sootblower is activated at regular intervals, generally between about 45-300 minutes. A correctly operating sootblowing system is of vital importance to the total economy of a mill, as the value of the consumed steam is high, and also as it is not uncommon that the mill has to stop its entire production of pulp for water washing the heating surfaces of the recovery furnace.
For a long time, the mills have desired to reduce the steam consumed by sootblowing. However, in principle this has been very difficult, as reduced steam consumption also has meant reduced soot removal efficiency. In many applications reduced soot removal efficiency is unacceptable, when you seek to attain high/secure/increased availability on the recovery furnace. Thus, there is a long-felt demand for a solution that makes it easy to save steam and simultaneously increase the efficiency of the sootblowing.
A principle description of a recovery furnace is found in WO 96/08677, which also discloses the use of sootblowers for removing heavy deposit, which is wholly or partially sintered, from the heating surfaces in a recovery furnace.
Several concepts of making the removal of soot more efficient have been presented and commercialized. In a first concept, the soot removal is governed by requirements. The operational intervals of the sootblowers are controlled from the calculated accumulation of soot on the heating surfaces. The saving of steam is achieved by breaks/pauses in the sootblowing, but often this is not acceptable to mills.
In a second concept, as depicted in U.S. Pat. No. 5,416,946, the sootblowers are operated with a reduced pressure (often in combination with a higher speed) during the return stroke or vice versa. As the pressure reduction between the steam source and the sootblowers is carried out at one common, single location, the sootblowers have to be operated one by one. This method saves steam, but simultaneously it reduces the efficiency of the sootblowing system somewhat. A similar solution is also known from US 20060065291, but intended for use in a different kind of boilers/furnaces, i.e. small sized.
In a third concept, the recovery furnace is divided into two (or more) sootblowing steam systems (front and back), where the sootblowers of one system can be operated independently of those in the other system. This method also has been combined with the first and second concepts above. The solution is complicated technically, as it includes much piping, new control stations and extensive programming to operate well from a process engineering point of view. Additionally, in practice the method is restricted by the existing construction of the trunk pipes that supply steam to the sootblowers. In practice, as a result of this restriction, the efficiency of the soot removal can be increased by at most about 30-50%.